Method of removing hemicellulose from wood pulp



April 8, 1952 J. MOK. LIMERICK 2,592,300

METHOD OF REMOVING HEMICELLULOSE FROM WOOD PULP Filed Sept. 10, 1946 3Sheets-Sheet l IN VEN TOR.

Jack M Limerick April 8, 1952 c UMERICK 2,592,300

METHOD OF REMOVING HEMICELLULOSE FROM WOOD PULP Filed Sept. 10, 1946 3Sheets-Sheet 2 r O Q .5 N 2; z 3 Q IN VEN TOR.

Jack K Limerick Ap 1952 J. MCK. LIMERICK METHOD OF REMOVINGHEMICELLULOSE FROM woon PULP Filed Sept. 10, 1946 s Sheets-Sheet 3 IN VEN TOR.

Jack 114 K- Limerick 'sidualJignin' and hemicellulose.

, Patented Apr. 8 1952 UNHT SAT 1' METHOD OF REMOVING HEMICEIJLULOSEFROM WOOD PULP J aclr McK. Limerick, Bathurst, New Brunswick, CanadaApplication September 10,1946, seriaiNo'r'eeacos 4 Claims.

Fig. l is an isometric diagram-illustrating a.

preferred arrangement of equipment'for carry- 'ing outtheimprovedprocess;

"Fig; 2 is an enlarged view of the left-hand side of Fig. 1; and

Fig. 3 is an'enlargedview ofthe'right-hand 'side of'Fig'. 1.

i Ordinarily wood pulp as it comes from the digester; and; almostregardless of the process by which the pulpihasbeenproduced; containsbe- '.tween 85 and 89% pure alpha cellulose-the bal- 1 'ancebeing made:up'of a small amount-of re- It is clear, IIJthGI'BfOIE; thatpurification would result in a n'yield'of'between 85'and 89% alphacellulose based z'on'the'original weight 'of the pulp. 'If' the treated"i pulp istobe usedifor the manufacture of'rayon, 1192" or 93% alphacontent is sufiicient but if the irpulp isto'be used for "the formationof clear "cellulose acetate, the'minimum permissible quali ity is 96%"alpha; ,Under the practice ofthe prior r-sart: the -law'of'diminishingreturns operates sewerely upon any attempt to raise'quality above 92%alpha cellulose and the yield drops off :Iabruptly.- For example: in oneinstance of cur '5? rent practice; there isan 80% yield with an alphacellulose content of 92% while when the quality is raised to 96% the"yield drops to 67%. The :"process of the'instant invention however, has-regularly produced qualities between 96 and 97% alpha cellulose withyields between 83 and 84%.

This is an enormous economic diiference.

Referring now-to Fig. 1 there is illustrated in the upper left-handcorner a largely convenctionalinstallationA for-vaporizing liquidchlolrinefrom tank carsand bringing it into the system. A control panelis illustrated at B, the ini- -tial chlorination stage atC, a freshwater washfollowing, chlorination at D, the first dilu- "tions'tep atE,the first'steep at F, the second .dilution at G, the second steep atH, thethird Ij 'dilutionlat I, thethird steep atJ, the fourth dilutionat'K; thefourthsteep at L, the thickening "step at M, three consecutivewashings at N. N andv N", the soaking stageat 0, the washing following"the soak' at P, the first hypochlorite stage at Q and Q, the washfollowing the first hypochlorite stage at R, the' second hypochlorite'stage' at S, S ancsfi 'the wash following the second hypochlorite stageat T, the sulphur dioxide stage at U, the screening followingthe'sulphurdioxide stage at V; wash-following the screening at W, the' final .stockchest: at 'X. Y represents the sulphur dioxide supply; and' Z thehypochlorite supply.

-Referringnow"'to*Fig. 2: washed, :unblea'ched sulphite I u1p' is storedin a stock chest I O at a consistency between 8 and 11%. A; pump l2,

through a line' l, sen'dsthe pulp to a mixing box-l6 where, through a,line l8, it is diluted-with fresh water 1' to ab'out 3-4%' consistency.The

-mixing box IE- is conventional and has ar-return line -20-to takeexcess pulp back to the steel;

chest 10.; -Dilute pulp leaves the box'l 6 through a line 22 and passes'throu'gh a mixer zd which also receives chlorine-from'plantA (shown inFig; 1) through a line '26 from control panel B.

The pulpand' chlorine then passin" sequence through towers 28-and 30.which constitutestation C. The chlorine used preferably 'amounts to" 65%of all chlorine to'beused. "It isareal bleaching step. A pump 32 takesthe pulp-from tank through a line 34xto a washer' and a thickener D.Onentering the washerD-the-pulp is diluted'to 1% consistency by meanssofa-line 38 supplied' by a' pump 40 which receives 'waterfromthejsuctionlinel'42 'ofrthe washer D. A freshwater shower '44"washes. the pulp as it "passes over the drum 46 of thewasherD; the

shower "44' being supplied by a'freshwater line 38. The pulppreferablyatabout 12.5% consistency leaves the washer D through a-'line50 'to a washer E. p

' Depending on the type of pulp it may be desirable not only to give thepulp a directchlorine bleach in towers 213 and 30 but to follow this bya hypochlorite bleach. In such-case the first hy- -pochlorite stage QQ'will be placed betweenthe -washer D and the washer E. It willalsqbe'desirable to add a washer. similar to washerDjbetween therelocated hypochlorite stagef'Q.Q ttr'idlthe washer E.- The latter (E)as will appear hereinafter, functions not. only to .wash the pulp "butprimarily to replace the liquor which is contai'nedin' the pulp as itapproachesthe washer with treatment liquor' and thus to avoid dilutionor "contamination of the treatment liquor.

. A wash step such as'D permits operation ofthe washer E tobepconcentrated on the replacement as, distinct from the l washingfunction.

-pn enteringthe washer E the pulp'is diluted to about 1% consistency bya' 'linefi l supplied by a pump 56 from'the suction-line"58' of the drum60 of the washer E. A shower 62 plays on the drum 60. The washer E firstthickens to 6-7% consistency and then dilutes the pulp to about 35%consistency. The pulp leaves the washer E through a line 64 which feedsa pump 66 which passes the pulp through a steeping tower F.

Since the actual removal of hemi-cellulose begins as the pulp leaveswasher E, it is essential A that the treating liquor supplied by line 82and shower 62 be not diluted by liquor carried over from washer D.Accordingly the volume of shower 62 is adjusted relative to the suctionon the drum 62 so that shower liquor is drawn completely through thepulp mat on the drum, thereby displacing the liquor in the pulp ahead ofthe shower. In order to maintain control, a pH controller 6| is placedin the line 58. The controller 6| acts, through any conventional line 63to control a valve 65 in the supply line of the shower 62. The volume ofshower 62, therefore, is regulated to maintain a pH in the line 58sufficiently high to guarantee that liquor from shower 62 is drawnthrough the pulp mat.

The line 64 contains a mixer 65 which receives dilute caustic (NaOH)from the control panel B through a line 61, and for pitch control,sodium phosphate maybe supplied through a line 61. This caustic supplyis used as a control to effect adjustment of the liquor concentration.As will later appear, when the process is seen as a whole, an adjustmentof concentration in the fresh liquor added at the last steep wouldrequire considerable time to affect all of the steeps. The mixer 65greatly shortens this lag. The vital factor to be controlled ishemi-cellulose concentration. Control is used early at the first steepfor the reason that only there can an excessive concentration develop.

From the tower F a pump 68 sends the pulp through a line 10 to a washerG. It is to be noted that the suction on the drum 12 of washer G drawsliquor through a line 14 and that this liquor by means of a pump 16 isused, first, by means of a line I8, to dilute the stock going to washerG to a consistency of 1%. The remaining portion of the liquor goesthrough a line 80 to the shower 62 of washer E and also, through a line82, to dilute the pulp leaving the washer E.

Since the total liquor removed from washer G through line I4 is greaterthan the total demand of lines 18 and 80 the excess liquor is bled offthrough a line 84 and is disposed of in a manner to be describedhereafter.

Washer G first thickens the pulp to a consistency of 1213% and thendilutes it back to 3-5%. The dilute stock leaves washer G through a line88 through which, by means of a pump 90, it is circulated through asecond steeping tower H. The pulp leaves the tower H through a line 92and by means of a pump 84 is delivered through a line 96 to a washer I.The suction drum 98 of the washer I delivers extracted liquor through aline I00 and this liquor is distributed by a pump I02 through a line I04to dilute the pulp in line 86 to 1% consistency and by means of a lineI06 the balance of the liquor extracted is delivered to a shower I08 onwasher G and through a line IIO the liquor in line I06 also dilutes thepulp leaving the washer G.

The drum 98 of washer I thickens the pulp to '7-8% consistency. The pulpis then re-diluted to a 3-5% consistency and leaves the washer I througha line I I2. The line II2 supplies a pump I I4 which feeds the pulpthrough a third steeping tower J. The pulp leaves the steeping tower Jthrough a line I I6 and by means of a pump H8 is supplied by a line I20to a washer K.

The liquor extracted by the drum I22 of washer K leaves the washerthrough a line I24 and by means of a pump I26 is delivered through aline I28 to intercept the pulp line I20 and to dilute the pulp to aconsistency of 1%; the balance of the liquor through a line I30 serves ashower I32 of the washer I and through a line I34 dilutes the pulp goingto the steeping tower J.

The drum I22 of the washer K thickens the pulp to a consistency of from10-12%. The pulp is then diluted to 4-5% consistency and leaves thewasher K through a line I36 which feeds a mixer I38. The mixer I38 alsoreceives, via the control panel B (Fig. 1), a line I40 which suppliessufficient fresh caustic liquor to dilute the pulp to 3-5% consistency.For a purpose to be described later the line I40 is passed through arefrigeration unit I42.

The pulp leaves the mixer I38 through a line I44 and is delivered bypump I46 to the fourth steeping tower L and leaves the tower L through aline I48. The line I48 feeds a pump I50 which sends the pulp to athickener M which brings the pulp to 22-24% consistency. The pulpentering the thickener M is diluted to 1% consistency by a line I52which is su plied by a line I54 which in turn receives its liquor fromthe suction line I56 of the drum I58 of the thickener M. The line I54supplies a shower I60 and, through a line I62, supplies dilution liquorto the washer K. The sole function of the thickener M is to remove asmuch liquor as possible from the pulp with no dilution. It will be notedthat there is no shower on the face of the drum and it may be desirableto mount press rolls over the drum to increase the liquor extraction.

The showers on Washers G, I and K are operated in a manner similar tothe shower on washer E, that is, the volume of these showers issufliciently relative to the vacuum on the drums to assure shower waterbeing drawn through the pulp mat. This effectively displaces theprevious liquor and provides sharp demarcation between the severalsteeping stages. To attain the same demarcation by increased thickeningbetween stages would involve costly apparatus and high powerconsumption. It is not necessary, at washers G, I and K to provide a pHcontroller as at washer E. Excessive flow of the shower 62 on washer Eultimately would mean a loss of chemicals to the sewer, as is clear inFig. 2. Excessive flow in the showers on washers G, I and K is notdiluted as in line 58 of washer E and is recovered in line 84 ashereafter described.

Agitation is provided in each of the steeping towers, F, H, J and L. Themost important agitating effect, however, occurs in the intermediatewashing steps where the extreme dilution, followed by thickening,followed by extremely agitated redilution serves thoroughly to break upany zones of hemi-cellulose saturated liquor around any pulp fibers.This assures maximum effectiveness of each steeping treatment. It alsoadds to the yield by minimizing the necessity for mechanical agitationin the steeping towers. Such agitation in a cold caustic suspension ofsubstantial consistency (3-5%) increases the hydration and solution ofalpha cellulose. Any alpha cellulose which is dissolved is lost, to thedetriment of yield.

Analysis of samples indicates that, on the average, each steepingtreatment accomplishes most"of the hemi-cellulose-dissolving 'ofwhich it"-is capableinfrom '-l0-'to l'minutes. Uniformity of product-however,-demands the safeguard of "longer--exposure;--hence;in practice; thesteep is -continued for-from 30 to 60 minutes.

'The pulp -leaving -the thickener 'M- is'diluted 'back' to-1%consistencythen goes" through aline fi HiGto a'washerm'thence'to a second'washe'r'N, thence'toathirdwasher N". These washers are conventional and arearranged for regular counter-currentwashing. A portion of the liquoroutput 'of washer- Nis bled-oil through'line I86 for a purpose 'tobedescribed" later.

At the washer N" a shower168 is'supplied with fresh water fromthe'1ine1l10 andstock leaving the washer N"; is diluted to about4%consistency by freshwater supplied by. a line I12. The line i H! .is:connected through an injector by 'means of' a line H4 with a'steam lineI16. A similar line and injector I18 connects the line I12 with"thesteam line I16.

The refrigeration unitl42 on the caustic line Mil 'has'already beenmentioned. A similar refrigeration unit 48 is placed in the line 38which brings fresh shower water to the shower of the washer andthickener'D: Still another refrigeration unit 48' placed' around theline. 80 going from washer E-to the first steeping tower F.

" "It is essential that the steeping in towers F, H,

flJy-and L be carried out at a relativelylow temperature preferablybetween 15 and degrees Centigrade. The caustic liquor entering thesystem through line I40 will usually be' quite warm "and therefrigeration unit I42 will operate 'to reduce-temperature to thedesired range. The fresh water supply at'most mills, particularlynorthern mills; during most of the year will be within the desiredrange. Therefore, the refrigeration unit 48' in the line 48 will onlyoccasionallybe used, and most of 'the'cooling'will be done by heatexch'angers using fresh water. The refrigeration unit '48 in line 8!will take care of any temper- "ature rise in" the treatingliquoroccurring during its passage from one steeping tower to the next.It greatly reduces the time lag of a temperature change effected by unit48. Indeed, fora good 1 part ofeachyear, particularlyat northern mills"unit"48' willdo the whole job'and unit'48 need not'be operated.

The-pulp leaving thelast washer N" is diluted to 34% consistencywith-hot-fresh water heated by the injectors H6 and H8. It goes'througha line I-"to the soaking tower 0 shown in Fig. 3. This tower is similarin proportions to the steeping-towers; F; H, J and L and, on theaverage, pulp will take halfan hour to go through the soaking -*-tank.-The pulp leavesthe tankthrough aline I82 1 "sand goes to a washer Pwhereit'receives a' shower 484 of fresh water.- Stock leaves the washer Pthrough a line I86 which takes it to amixer I88 -"-'where it is treatedwith hypochlorite solution i'brought tothemixer I 88 through a line I 90from "thehypochlorite supply" Z (Fig. '1) through the control panel Balso shown in Fig. 1. If desired, caustic sufficient to maintainalkalinity-during --the hypochlorite bleach may also be supplied tothe-mixer I88. Stock plus hypochlorite leaves the mixer I88 and througha line I92 goes to a hypochlorite tower Q thence through a line I94 to asecond. hypochlorite tower Q The stock leaves the tower Q through alinelSS which takes it to a washer R where it receives a fresh watershower "198." From the washer R the stock leaves through atlineiflfi toa stock chest 292 where it is stored and agitated by conventionalagitators 2%.

A pump 206- removes from the chest zoz and,

- through a line 208 delivers the stock to mixer 2 l0.- The mixer 21 Il-receiveshypochloritesolution through a line 212 which, through-control"panel B, receives hypochlorite solutionifromthe-bleach supply Z. A line2|3 supplies fresh water to the mixer 210- a a means ofcontrollingconsistency. From the mixer m the stock proceedsYthroughparallel lines 214, 2 I6 and M8 to parallel bleaching towers S, S and S"respectively.

It will be noted that pulp is delivered to the top of towers S; S" andS". '-When "each' tower is filled; the pulpis' circulated in thetowereand "samples are periodically taken. 'when-thew-samplingind-icatesthat the desiredqualities have'been attained in'any tower, thetower is dumped into i a chest'220 where it-is'agitated'by conventional.a'gitators222. ,ThiSfiS essentially a batch operation'of each towerand,throughthisopportunity for close control, extreme uniformity of -the'end a 1% solution of the pulp in a'standardized-cup- -ramm'oniumsolution. For ordinary-discose work, the solution viscosity shouldbefifrom 20 30 centipoise; for acetates or nitrates the viscosity shouldbe 30 .40 centipoise. The oxidationoccurring in the hypochlorite stagetends'to shorten'the alpha cellulose molecule and thus to lower thesolution viscosity .of the pulp. This should" be done on thealkalineside, with "pH' of 8 or..9,preferably9. Carried too far, thisprocess produces cxy-cellulose to the detriment of yield. .Suchdegradation of the pulp varies'directly' with'c'onsistency which,accordingly, must be controlled. A pump 224 takes stock'from'the' chest220*and through a line.226 delivers it to a washer'Twh'ere it receives afresh water sh0we'r228. Stockleaves the washer Tthrough a line 233 to amixe'ri1232. The mixer also receives sulphur dioxideithrough alinei234which is supplied via the control panel B from the SOzsupply Y. "Fromthe mixer 232 stock goes through ailine 235 to a treatment tower .U. Itleaves the tower U through a line 238 to a consistency regulator 2M]where it is diluted with fresh water through a line 242 and also inpartwith the 'efiluent from a washer W through a line 2 14., From theconsistency regulator 240s'tock goes thru' lines 246 to a set ofconventional'fiat screens V.

.ates substantially as a thickener to remover-the extreme dilution(0.28% consistency) encountered at the'fiat screens.

Accepted stock from the-fiat screens' 'V-t-goes through a line 248-to achest-250 from which a pump 252 carries the stock through a line 254 tohot this process is a juxta-position of kraft and sulphite pulp mills'since the system is demon- Percent Sodium hydroxide 7.00 Sodiumsulphide 4.25 Sodium carbonate Sodium sulphate With such an installationthe bleed-off through lines 84 and I66 may be evaporated back to theconcentration of the standard cooking liquor and may be used in thedigestor. It is also permissible to use this liquor directly in thedigestor. When this is done, the amount of black liquor normally used todilute the digestor liquor is reduced. The presence of between 2 and 5%hemi-cellulose in the reclaimed white liquor, when used to make up aportion 'of the digestor charge, provides a species of protectivecolloid exercising a buffer effect against too drastic an action on thecaustic of the wood. Moreover the addition of hemi-cellulose adds to theMullen test of paper made from pulp thus digested.

It will, of course, be noted that the liquor from the last steepingstage in tower L goes from thickener M back to washer K and, therefore,recirculates in large part through the tower L. The portion of thisliquor going to the shower ISO is delivered as shower and dilution waterto the washer I which supplies the third steeping tower J and freshcaustic is added only at the fourth steep L.

Liquor containing the largest percentage of hemi-cellulose leaves thesystem through line 84 from the washer G. The rate of bleeding isadjusted proportionally to the rate of addition of fresh liquor so as tolimit the maximum concentration of hemi-cellulose in the steeping liquorto not more than 5% and preferably to 3% in first steeping tower F andto 2% or less in the other steeping towers. This limitation, combinedwith the relatively low temperature, appears to be critical in securingthe combination of high yield and. high quality. As the hemi-cellulosecontent of the steeping liquor increases it becomes progres sively moredifficult to dissolve hemi-cellulose in the liquor. Also, there is adegradation both of caustic soda and of sodium sulphide with an increasein the percent of sodium carbonate. There is also a definite increase inviscosity which in turn tends to slow the process of dissolvinghemi-cellulose. While 3% hemi-cellulose content in the liquor isbelieved to be optimum from all standpoints, it is still feasible topermit the concentration to rise to 5%. Beyond 5%, however, the abilityof the liquor to dissolve hemi-cellulose drops rapidly, viscosityincreases sharply and the degradation of caustic and of sodium sulphideis so marked that the liquor would not be useable in the kraft process.

The soaking step carried out in tower O is of particular advantage. Itaffords an opportunity for virtually complete removal, not only ofcaustic but also of the sodium sulphide in the kraft white liquor. Ifany substantial quantity of sodium sulphide were permitted to remain itwould negative a corresponding portion of the hypochlorite and greatlyincrease bleaching costs. In addition, in practice, the use of thissoaking step definitely increases quality by removing a small amount ofhemi-cellulose.

The caustic steepings carried out by this process proceed on acounter-current principle, that is,

fresh incoming pulp with the greatest proportion of availablehemi-cellulose. is subjected to the action of liquor which contains theheaviest permissible concentration of dissolved hemi-cellulose while themost potent liquor with the lowest concentration of hemi-cellulose isused to treat the pulp with the least proportion available ofhemi-cellulose. This is infinitely more eflicient than a singleprolonged steeping. The repeated steps are also important because of theopportunity thus afforded in dilution, thickening and re-dilution forbreaking up localized concentrations of hemi-cellulose adjacent theindividual fibers and thus permitting the great mass of liquor to bemore fully effective.

As used in the claims the term stage is intended to include the completecycle of each event. That is, with reference to a steeping stage, theterm stage includes dilution, thickening with concomitant displacementof the previous liquor, redilution and then steeping. The term step isused to designate any particular operation such as dilution orthickening or redilution or steeping.

I claim:

1. A multi-stage continuous process for purifying pulp each stage ofwhich consists of the consecutive steps of providing a slurry of pulpand alkaline treating liquor in which the pulp,

by weight, is from approximately 3 to approximately 5%, diluting theslurry to a pulp consistency of approximately 1% by the addition ofalkaline treating liquor, then thickening the slurry to a consistency ofapproximately 6 to approximately 12% while simultaneously displacing thefirst alkaline treating liquor with a second, stronger alkaline treatingliquor, then diluting the slurry to approximately 3 to approximately 5%pulp consistency by the addition of more of the second, strongeralkaline treating liquor, then advancing the thus diluted slurrycontinuously along a closed, predetermined path for a period ofapproximately fifteen to approximately sixty minutes, then repeating allof the above steps in three to four stages using consecutively strongerliquors for the final dilution step in each stage, the final dilutionliquor in each stage coming from the thickening step of the nextsucceeding stage, continuously removing a predetermined proportion ofliquor from one of the early stages, and continuously adding anequivalent amount of fresh liquor at the final dilution step of the laststage and regulating the rates of addition of fresh liquor and theremoval of spent liquor to maintain the concentration of hemi-cellulosein the liquors below 5%, the several diluting steps being carried outwith agitation to break up localized concentrations of dissolvedhemi-cellulose adjacent the individual pulp fibers.

2. The process set forth in claim 1 in which the pulp following theseveral stages therein set forth is washed and then steepedapproximately fifteen to approximately sixty minutes in hot fresh water.

3. The process set forth in claim 1 in which the stages therein setforth are preceded by a bleaching treatment with a fresh water washingstage intermediate the bleaching treatment and the said stages set forthin claim 1, said bleaching treatment using an agent selected from thegroup of chlorine and hypochlorites.

4. The process set forth in claim 3 in which the pulp following thecompletion of the process of claim 3 is given a fresh water wash andthen 9 is steeped for approximately fifteen to approximately sixtyminutes in hot ireshja'veter.

JACK McKi LIMERICK. REFERENCES omen I The following references are of{record in the file of this patent: A

UNITED STATES PA IEENTS Number Name I Date 1,632,802 Richter June 21,1927 1,742,218 Richter Jan. 7, 1930 1,798,987 Rue 1Ma1-. 31, 19311,801,782 Richter Apr. 21, 1931 1,829,378 Thiriet Oct. 27, 1931 15 v 10Number Name Date 1,906,885 Richter May 2, 1933 1,933,609 Wagner Nov. 7,1933 2,041,666 Richter May 19, 1936 2,219,174 Richter July 15, 1941 2324,230 Olsen July 13, 1943 2,363,684 Richter Aug. 28, 1945 2,385,259Collings Sept. 18, 1945 1 OTHER REFERENCES Cellulose and CelluloseDerivatives by Ott, publish'ed by Interscience Publishers. Inc., NewYork (1943), pages 274, 511 to 515, 601' to 803, 813 and 814.

1. A MULTI-STAGE CONTINUOUS POROCESS FOR PURIFYING PULP EACH STAGE OFWHICH CONSISTS OF THE CONSECUTIVE STEPS OF PROVIDING A SLURRY A PULP ANDALKALINE TREATING LIQUID IN WHICH THE PULP, BY WEIGHT, IS FROMAPPROXIMATELY 3 TO APPROXIMATELY 5%, DILUTING THE SLURRY TO A PULPCONSISTENCY OF APPROMIMATELY 1% BY THE ADDITION OF ALKALINE TREATINGLIQUOR, THEN THICKENING THE SLURRY TO A CONSISTENCY OF APPROXIMATELY 6TO APPROXIMATELY 12% WHILE SIMULTANEOUSLY DISPLACING THE FIRST ALKALINETREATING LIQUOR WITH A SECOND, STRONGER ALKALINE TREATING LIQUOR, THENDILUTING THE SLURRY TO APPROXIMATELY 3 TO APPROMIMATELY 5% PULPCONSISTENCY BY THE ADDITION OF MORE OF THE SECOND, STRONGER ALKALINETREATING LIQUOR, THEN ADVANCING THE THUS DILUTED SLURRY CONTINUOSLYALONG A CLOSED, PREDETERMINED PATH FOR A PERIOD OF APPROXIMATELY FIFTEENTO APPROXIMATELY SIXTY MINUTES, THEN REPEATING ALL OF THE ABOVE STEPS INTHREE TO FOUR STAGES USING CONSECUTIVELY STRONGER LIQUORS FOR THE FINALDILUTION STEP IN EACH STAGE, THE FINAL DILUTION LIQUOR IN EACH STAGECOMING FROM THE THICKENING STEP OF THE NEXT SUCCEEDING STAGE,CONTINUOUSLY REMOVING A PREDETERMINED PROPORTION OF LIQUOR FROM ONE OFTHE EARLY STAGES, AND CONTINUOUSLY ADDING AN EQUIVALENT AMOUNT OF FRESHLIQUOR FROM ONE OF THE STEP OF THE LAST STAGE AND REGULATING THE RATESOF ADDITION OF FRESH LIQUOR AND THE REMOVAL OF SPENT LIQUOR TO MAINTAINTHE CONCENTRATION OF HEMI-CELLULOSE IN THE LIQUORS BELOW 5%, THE SEVERALDILUTING STEPS BEING CARRIED OUT WITH AGITATION TO BREAK UP LOCALIZEDCONCENTRATIONS OF DISSOLVED HEMI-CELLULOSE ADJACENT THE INDIVIDUAL PULPFIBERS.